1. Technical Field of the Invention
The present invention relates generally to tire inflation pressure sensing apparatuses or sensors. More particularly, the invention relates to a direct-type tire inflation pressure sensing apparatus which is configured to be supplied with electrical power via wireless communication.
2. Description of the Related Art
Conventional direct-type tire inflation pressure sensing apparatuses generally include a transmitter and a receiver. (For example, Japanese Patent No. 3212311, an English equivalent of which is U.S. Pat. No. 5,602,524, discloses such an apparatus.)
The transmitter is directly installed to a wheel of a vehicle and includes a pressure sensor working to sense an inflation pressure of a tire mounted on the wheel. The transmitter is configured to send out a pressure signal representative of the inflation pressure of the tire sensed by the pressure sensor.
The receiver is installed to the body of the vehicle and includes an antenna. The receiver is configured to receive the pressure signal from the transmitter through the antenna and determine the inflation pressure of the tire based on the received pressure signal.
In the above arrangement, the wireless communication between the transmitter and the receiver may be achieved by applying an interrogator/transponder method.
According to the interrogator/transponder method, the receiver (i.e., the interrogator) sends out through the antenna thereof a radio wave for charging the transmitter (i.e., the transponder) with electrical power. After being charged via the radio wave, the transmitter is then activated by the charged electrical power and works to send out the pressure signal toward the receiver.
Such an interrogator/transponder method is generally used in the filed of wireless ID cards. The application of the method to the direct-type tire inflation pressure sensing apparatuses provides an advantage of supplying electrical power to the transmitter without any battery being provided in the transmitter.
However, in the case of supplying electrical power by the method, the possible range of angular position of the transmitter for charge, within which the level of the radio wave received by the receiver is above a necessary level for charging the transmitter with electrical power, is very small when the transmitter rotates with rotation of the wheel.
Further, in the above case, the electrical power available for the transmitter is considerably smaller than in the case of supplying electrical power by a battery. Consequently, the level of the pressure signal sent out from the transmitter becomes considerably lower than in the letter case. As a result, the possible range of angular position of the transmitter for reception, within which the level of the pressure signal received by the receiver is above a necessary level for accurate recognition of the pressure signal, also becomes considerably small.
Accordingly, the possible range of angular position of the transmitter for communication, within which levels of the radio wave received by the transmitter and the pressure signal received by the receiver are above a necessary level for establishing a desired communication between the transmitter and the receiver, is very small.
FIGS. 9A-9B illustrates the position of the transmitter relative to the antenna of the receiver. In the figures, the antenna J1 of the receiver is installed to a fender J3 of the vehicle body such that a vertical centerline C-C of the wheel J4 passes through the antenna J1.
In this case, when the transmitter J2 rotates to, for example, an angular position where the transmitter J2 is opposed to the antenna J1 with the wheel J4 positioned therebetween, both the radio wave for electrical power charge from the antenna J1 and the pressure signal from the transmitter J2 cannot pass through the wheel J4 which is made of a metal material.
According to the results of an investigation by the inventors, in this case, the possible range of angular position of the transmitter for communication (to be referred to as possible range for communication hereinafter) may be defined as the angular range A-O-B of about 80°, which spreads out from the shaft of the wheel J4 and is symmetrical with respect to the vertical centerline C-C.
In other words, when the transmitter J2 is within the above angular range A-O-B, the transmitter J2 can reliably receive the radio wave for electrical power charge from the antenna J1 and the receiver can accurately recognize the pressure signal received thereby through the antenna J1.
Accordingly, it is required for the transmitter J2 to be charged with electrical power and triggered to send out the pressure signal toward the antenna J1 when it rotates in the angular range A-O-B. Further, it is required for the transmitter J2 to complete the sending out of the pressure signal before it passes over the angular range A-O-B.
However, it is difficult for the receiver to accurately determine the angular position of the transmitter J2 when the transmitter J2 rotates together with the wheel J4.
Moreover, to accurately determine the angular position of the transmitter J2, additional sensors or devices are required. However, this will increases the manufacturing cost and electrical power consumption of the tire inflation pressure sensing apparatus.